Lithographic apparatus and device manufacturing method

ABSTRACT

A lithographic apparatus includes a patterning device that patterns a projected beam. The patterning device includes an array of cells that contain a polar fluid, a non-polar fluid, and an electrode. A potential difference across the electrode and the polar fluid causes displacement of the non-polar fluid. Based on a difference in refractive index between the polar fluid and the non-polar fluid, a beam of light which passes through the cell will have its phase changed in dependence on the relative thickness on the polar and non-polar fluids and on their refractive indices.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic apparatus and a devicemanufacturing method.

2. Related Art

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. The lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs), flatpanel displays and other devices involving fine structures. In aconventional lithographic apparatus, a patterning device, which isalternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern corresponding to an individual layer of theIC (or other device). This pattern can be imaged onto a target portion(e.g., part of one or several dies) on a substrate (e.g., a siliconwafer or glass plate) that has a layer of radiation-sensitive material(e.g., resist). Instead of a mask, the patterning device may comprise anarray of individually controllable elements that generate the circuitpattern.

In general, a single substrate will contain a network of adjacent targetportions that are successively exposed. Known lithographic apparatusinclude steppers, in which each target portion is irradiated by exposingan entire pattern onto the target portion in one go, and scanners, inwhich each target portion is irradiated by scanning the pattern throughthe projection beam in a given direction (e.g., the “scanning”direction), while synchronously scanning the substrate parallel or antiparallel to this direction.

The individually controllable elements that generate the pattern in thepatterning device may take several forms. Conventionally, there are twotypes of pure-phase modulating individually controllable elements. Afirst type is based on a reflective layer placed on a compliant layer. Asecond type is based on individual micro-mirrors that can be moved in adirection perpendicular to their reflecting surface.

However, both of these types of individually controllable elements aredifficult to manufacture and are not efficient in accurate patternplacement when trying to also maintain pattern fidelity. This is largelybecause of they require mechanical motion for arrays of very smallindividually controllable elements.

Therefore, what is needed is a system and method that can provide a purephase-shifting array of individually controllable elements that iscapable of accurate edge placement without loss of pattern fidelity.

SUMMARY OF THE INVENTION

One embodiment of the present invention provides a lithographicapparatus including an illumination system for supplying a projectionbeam of radiation, a patterning array having individually controllableelements that impart the projection beam with a pattern, a substratetable for supporting a substrate during an exposure process, and aprojection system for projecting the patterned beam onto a targetportion of the substrate. The individually controllable elements eachcomprise a cell containing a polar fluid, a non-polar fluid, and avoltage source arranged to apply selectively an electric field acrossthe cell for voltage-controlled displacement of the non-polar fluidwithin the cell. The polar fluid and the non-polar fluid havesubstantially the same transmissivity, but different refractive indices.

Through the use of an array of cells containing polar and non-polarfluids little or no mechanical motion is required. Thus, there is noneed for actuators, which can be bulky and difficult to controlaccurately. Each cell receives a voltage across different parts of thecell. The extent of displacement of non-polar fluid may then becontrolled by this voltage. The ratio of polar to non-polar fluid in theline of the projection beam changes the phase of the part of theprojected beam associated with the cell by a desired amount as it passesthrough the cell. Because the voltage can be controlled accurately, thedisplacement of the non-polar fluid can be changed accurately, therebygiving a pure phase-shifting individually controllable element that iscapable of accurate edge placement without loss of pattern.

The voltage source is adapted to change the voltage level to give adesired level of attraction on the polar fluid, thereby displacing thenon-polar fluid by a desired amount. In this way, the attraction of thepolar fluid towards one part of the cell may cause the displacement ofthe non-polar fluid towards a different part of the cell.

The relative properties of the polar and non-polar fluids are chosen tochange the phase of the projected beam of radiation as desired. This isdone by having different refractive indices for the different fluids. Asthe beam passes first into one fluid and then the other, the change inrefractive index causes the properties of the beam to change. The phasechange of the part of the beam that passes through the cell isproportional to the difference in refractive index of the fluids throughwhich it passes.

In one example, the polar fluid is water and the non-polar fluid is oil.In this example, these fluids are chosen because they are immiscible andso the movement of one will displace the other. These fluids are alsosimple and inexpensive to acquire and/or maintain.

The cell may comprise a reflective surface on the opposite surface fromthe radiation entry surface, such that the part of the projected beamassociated with the cell travels through the cell twice. Directing theprojected beam through the cell twice via a reflective surface canresult in the projected beam phase being changed by twice as much withthe same displacement of the non-polar fluid as a cell without thereflective surface, which simply transmits the projected beam through itonce.

A second embodiment of the present invention provides a devicemanufacturing method comprising the steps of providing a substrate,providing a projection beam of radiation using an illumination system,using a patterning array comprising individually controllable elementsto impart the projection beam with a pattern, projecting the patternedbeam of radiation onto a target portion of the substrate, andcontrolling the phase of the part of the beam associated with eachelement by directing it through a cell containing layers of polar andnon-polar fluids, the fluids having substantially the sametransmissivity, but different refractive indices and having adjustablerelative layer thicknesses.

Further embodiments, features, and advantages of the present inventions,as well as the structure and operation of the various embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1 depicts a lithographic apparatus according to one embodiment ofthe present invention.

FIGS. 2 and 3 depict an individually controllable element in first andsecond states according to one embodiment of the present invention.

The present invention will now be described with reference to theaccompanying drawings. In the drawings, like reference numbers mayindicate identical or functionally similar elements.

DETAILED DESCRIPTION OF THE INVENTION

Overview and Terminology

The term “array of individually controllable elements” as here employedshould be broadly interpreted as referring to any device that can beused to endow an incoming radiation beam with a patterned cross-section,so that a desired pattern can be created in a target portion of thesubstrate. The terms “light valve” and “Spatial Light Modulator” (SLM)can also be used in this context. Examples of such patterning devicesare discussed below.

A programmable mirror array may comprise a matrix-addressable surfacehaving a viscoelastic control layer and a reflective surface. The basicprinciple behind such an apparatus is that, for example, addressed areasof the reflective surface reflect incident light as diffracted light,whereas unaddressed areas reflect incident light as undiffracted light.Using an appropriate spatial filter, the undiffracted light can befiltered out of the reflected beam, leaving only the diffracted light toreach the substrate. In this manner, the beam becomes patternedaccording to the addressing pattern of the matrix-addressable surface.

It will be appreciated that, as an alternative, the filter may filterout the diffracted light, leaving the undiffracted light to reach thesubstrate. An array of diffractive optical micro electrical mechanicalsystem (MEMS) devices can also be used in a corresponding manner. Eachdiffractive optical MEMS device can include a plurality of reflectiveribbons that can be deformed relative to one another to form a gratingthat reflects incident light as diffracted light.

A further alternative embodiment can include a programmable mirror arrayemploying a matrix arrangement of tiny mirrors, each of which can beindividually tilted about an axis by applying a suitable localizedelectric field, or by employing piezoelectric actuation means. Onceagain, the mirrors are matrix-addressable, such that addressed mirrorswill reflect an incoming radiation beam in a different direction tounaddressed mirrors; in this manner, the reflected beam is patternedaccording to the addressing pattern of the matrix-addressable mirrors.The required matrix addressing can be performed using suitableelectronic means.

In both of the situations described here above, the array ofindividually controllable elements can comprise one or more programmablemirror arrays. More information on mirror arrays as here referred to canbe gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193,and PCT patent applications WO 98/38597 and WO 98/33096, which areincorporated herein by reference in their entireties.

A programmable LCD array can also be used. An example of such aconstruction is given in U.S. Pat. No. 5,229,872, which is incorporatedherein by reference in its entirety.

It should be appreciated that where pre-biasing of features, opticalproximity correction features, phase variation techniques and multipleexposure techniques are used, for example, the pattern “displayed” onthe array of individually controllable elements may differ substantiallyfrom the pattern eventually transferred to a layer of or on thesubstrate. Similarly, the pattern eventually generated on the substratemay not correspond to the pattern formed at any one instant on the arrayof individually controllable elements. This may be the case in anarrangement in which the eventual pattern formed on each part of thesubstrate is built up over a given period of time or a given number ofexposures during which the pattern on the array of individuallycontrollable elements and/or the relative position of the substratechanges.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as, for example, the manufacture of DNA chips,MEMS, MOEMS, integrated optical systems, guidance and detection patternsfor magnetic domain memories, flat panel displays, thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist) or a metrology or inspection tool.Where applicable, the disclosure herein may be applied to such and othersubstrate processing tools. Further, the substrate may be processed morethan once, for example in order to create a multi-layer IC, so that theterm substrate used herein may also refer to a substrate that alreadycontains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of 365, 248, 193, 157 or 126 nm) and extremeultra-violet (EUV) radiation (e.g. having a wavelength in the range of5–20 nm), as well as particle beams, such as ion beams or electronbeams.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection systems, includingrefractive optical systems, reflective optical systems, and catadioptricoptical systems, as appropriate, for example, for the exposure radiationbeing used, or for other factors such as the use of an immersion fluidor the use of a vacuum. Any use of the term “lens” herein may beconsidered as synonymous with the more general term “projection system.”

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents for directing, shaping, or controlling the projection beam ofradiation, and such components may also be referred to below,collectively or singularly, as a “lens.”

The lithographic apparatus may be of a type having two (e.g., dualstage) or more substrate tables (and/or two or more mask tables). Insuch “multiple stage” machines the additional tables may be used inparallel, or preparatory steps may be carried out on one or more tableswhile one or more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index (e.g.,water), so as to fill a space between the final element of theprojection system and the substrate. Immersion liquids may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the first element of the projection system.Immersion techniques are well known in the art for increasing thenumerical aperture of projection systems.

Further, the apparatus may be provided with a fluid processing cell toallow interactions between a fluid and irradiated parts of the substrate(e.g., to selectively attach chemicals to the substrate or toselectively modify the surface structure of the substrate).

Lithographic Projection Apparatus

FIG. 1 schematically depicts a lithographic projection apparatus 100according to an embodiment of the invention. Apparatus 100 includes atleast a radiation system 102, an array of individually controllableelements 104, an object table 106 (e.g., a substrate table), and aprojection system (“lens”) 108.

Radiation system 102 can be used for supplying a projection beam 110 ofradiation (e.g., UV radiation), which in this particular case alsocomprises a radiation source 112.

An array of individually controllable elements 104 (e.g., a programmablemirror array) can be used for applying a pattern to projection beam 110.In general, the position of the array of individually controllableelements 104 can be fixed relative to projection system 108. However, inan alternative arrangement, an array of individually controllableelements 104 may be connected to a positioning device (not shown) foraccurately positioning it with respect to projection system 108. As heredepicted, individually controllable elements 104 are of a reflectivetype (e.g., have a reflective array of individually controllableelements).

Object table 106 can be provided with a substrate holder (notspecifically shown) for holding a substrate 114 (e.g., a resist-coatedsilicon wafer or glass substrate) and object table 106 can be connectedto a positioning device 116 for accurately positioning substrate 114with respect to projection system 108.

Projection system 108 (e.g., a quartz and/or CaF₂ lens system or acatadioptric system comprising lens elements made from such materials,or a mirror system) can be used for projecting the patterned beamreceived from a beam splitter 118 onto a target portion 120 (e.g., oneor more dies) of substrate 114. Projection system 108 may project animage of the array of individually controllable elements 104 ontosubstrate 114. Alternatively, projection system 108 may project imagesof secondary sources for which the elements of the array of individuallycontrollable elements 104 act as shutters. Projection system 108 mayalso comprise a micro lens array (MLA) to form the secondary sources andto project microspots onto substrate 114.

Source 112 (e.g., an excimer laser) can produce a beam of radiation 122.Beam 122 is fed into an illumination system (illuminator) 124, eitherdirectly or after having traversed conditioning device 126, such as abeam expander 126, for example. Illuminator 124 may comprise anadjusting device 128 for setting the outer and/or inner radial extent(commonly referred to as σ-outer and σ-inner, respectively) of theintensity distribution in beam 122. In addition, illuminator 124 willgenerally include various other components, such as an integrator 130and a condenser 132. In this way, projection beam 110 impinging on thearray of individually controllable elements 104 has a desired uniformityand intensity distribution in its cross-section.

It should be noted, with regard to FIG. 1, that source 112 may be withinthe housing of lithographic projection apparatus 100 (as is often thecase when source 112 is a mercury lamp, for example). In alternativeembodiments, source 112 may also be remote from lithographic projectionapparatus 100. In this case, radiation beam 122 would be directed intoapparatus 100 (e.g., with the aid of suitable directing mirrors). Thislatter scenario is often the case when source 112 is an excimer laser.It is to be appreciated that both of these scenarios are contemplatedwithin the scope of the present invention.

Beam 110 subsequently intercepts the array of individually controllableelements 104 after being directing using beam splitter 118. Having beenreflected by the array of individually controllable elements 104, beam110 passes through projection system 108, which focuses beam 110 onto atarget portion 120 of the substrate 114.

With the aid of positioning device 116 (and optionally interferometricmeasuring device 134 on a base plate 136 that receives interferometricbeams 138 via beam splitter 140), substrate table 106 can be movedaccurately, so as to position different target portions 120 in the pathof beam 110. Where used, the positioning device for the array ofindividually controllable elements 104 can be used to accurately correctthe position of the array of individually controllable elements 104 withrespect to the path of beam 110, e.g., during a scan. In general,movement of object table 106 is realized with the aid of a long-strokemodule (course positioning) and a short-stroke module (finepositioning), which are not explicitly depicted in FIG. 1. A similarsystem may also be used to position the array of individuallycontrollable elements 104. It will be appreciated that projection beam110 may alternatively/additionally be moveable, while object table 106and/or the array of individually controllable elements 104 may have afixed position to provide the required relative movement.

In an alternative configuration of the embodiment, substrate table 106may be fixed, with substrate 114 being moveable over substrate table106. Where this is done, substrate table 106 is provided with amultitude of openings on a flat uppermost surface, gas being fed throughthe openings to provide a gas cushion which is capable of supportingsubstrate 114. This is conventionally referred to as an air bearingarrangement. Substrate 114 is moved over substrate table 106 using oneor more actuators (not shown), which are capable of accuratelypositioning substrate 114 with respect to the path of beam 110.Alternatively, substrate 114 may be moved over substrate table 106 byselectively starting and stopping the passage of gas through theopenings.

Although lithography apparatus 100 according to the invention is hereindescribed as being for exposing a resist on a substrate, it will beappreciated that the invention is not limited to this use and apparatus100 may be used to project a patterned projection beam 110 for use inresistless lithography.

The depicted apparatus 100 can be used in four preferred modes:

-   -   1. Step mode: the entire pattern on the array of individually        controllable elements 104 is projected in one go (i.e., a single        “flash”) onto a target portion 120. Substrate table 106 is then        moved in the x and/or y directions to a different position for a        different target portion 120 to be irradiated by patterned        projection beam 110.    -   2. Scan mode: essentially the same as step mode, except that a        given target portion 120 is not exposed in a single “flash.”        Instead, the array of individually controllable elements 104 is        movable in a given direction (the so-called “scan direction”,        e.g., the y direction) with a speed v, so that patterned        projection beam 110 is caused to scan over the array of        individually controllable elements 104. Concurrently, substrate        table 106 is simultaneously moved in the same or opposite        direction at a speed V=Mv, in which M is the magnification of        projection system 108. In this manner, a relatively large target        portion 120 can be exposed, without having to compromise on        resolution.    -   3. Pulse mode: the array of individually controllable elements        104 is kept essentially stationary and the entire pattern is        projected onto a target portion 120 of substrate 114 using        pulsed radiation system 102. Substrate table 106 is moved with        an essentially constant speed such that patterned projection        beam 110 is caused to scan a line across substrate 106. The        pattern on the array of individually controllable elements 104        is updated as required between pulses of radiation system 102        and the pulses are timed such that successive target portions        120 are exposed at the required locations on substrate 114.        Consequently, patterned projection beam 110 can scan across        substrate 114 to expose the complete pattern for a strip of        substrate 114. The process is repeated until complete substrate        114 has been exposed line by line.    -   4. Continuous scan mode: essentially the same as pulse mode        except that a substantially constant radiation system 102 is        used and the pattern on the array of individually controllable        elements 104 is updated as patterned projection beam 110 scans        across substrate 114 and exposes it.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Exemplary Imaging Arrangements

FIG. 2 shows an individually controllable element 200 in a first state,according to one embodiment of the present invention. In thisembodiment, individually controllable element 200 is in the form of acell 210 containing a polar fluid 212, for example water, and anon-polar fluid 214, for example oil. Cell 210 also contains anelectrode 218, which can have a potential difference applied between itand polar fluid 212. There is also an insulating layer 216 to insulateelectrode 218 from fluids 212, 214.

In this embodiment, an incoming projected radiation beam 202 (202 a, 202b, 202 c, . . . ) is transmitted to pass through polar fluid 212 andnon-polar fluid 214. Passing through fluids 212 and 214 changes a phaseof projected radiation beam 202.

In one example, the phase is changed in accordance with the followingformula:Δφ=2πnd/λ(n1−n2)whereΔφ=change in phase;d=the thickness of the non-polar fluid 214;λ=wavelength of the radiation beam 202;n1=refractive index of the non-polar fluid 214; andn2=refractive index of the polar fluid 212.

For example, if d=1 μm, n1−n2=0.1, and λ=193 nm, the phase differencewill be approximately π. In a system using a reflective individuallycontrollable element 200, the phase difference will be 2π.

FIG. 3 shows cell 210 in a second state, which is when a potentialdifference is applied across electrode 218 and polar fluid 212. Thepotential difference causes polar fluid 212 to be attracted to electrode218. This causes displacement of non-polar fluid 214 as shown in thefigure. This process is known as “electrowetting” because the insulatingsurface becomes more or less “wettable” depending on the voltage appliedacross cell 210. For example, electrode 218 changes the wettability ofsurface layer 216 basically determining a contact angle between surfacelayer 216, and thus a shape of a fluid interface. The counter“electrode” is polar fluid 212.

In this example, projected radiation beam 202 c is projected through adifferent ratio of polar fluid 212 to non-polar fluid 214 as compared toprojected radiation beam 202 c in FIG. 2. As noted in the formula above,in this example the phase change will change in proportion with thevalue of d, that is, the thickness of non-polar fluid 214.

Projected radiation beam 202 a, on the other hand, no longer passesthrough non-polar liquid 214 (or through as much non-polar liquid 214),so the phase change should be dependent on its passage through polarliquid 212.

FIG. 3 also shows an alternative embodiment having a different inputdirection for a projected radiation beam 306. In this embodiment, onlyprojection radiation beam 306 would be passing through cell 210, and notprojection radiation beam 202. A relative thickness of non-polar fluid214 and polar fluid 212 can be achieved in order to change the phase ofprojected radiation beam 306 by a different amount.

In another alternative embodiment, when cell 210 is used as a reflectiveindividually controllable element 200, the reflective surface can be onan opposite face 320 of cell 210 from a face 322 in which projectedradiation beam 202 is input. In one example, this allows projectionradiation beam 202 to pass twice through cell 210. It is to beappreciated that any internal surface of cell 210 can be the reflectivesurface because projected radiation beam 202 is projected throughmaterials with different refractive indices, which changes the phase ofthe beam, not just the order in which it passes through them. Forexample, in this embodiment the reflective surface forms part ofinsulating layer 216. It is to be appreciated other areas of cell 210can also be or include the reflective surface, which are allcontemplated within the scope of the present invention.

It is to be appreciated alternative embodiments may include havingelectrode 218 on a different surface of cell 210 and in differentconfigurations, as long as non-polar fluid 214 and polar fluid 212 arerelatively displaced by the potential difference described above.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A lithographic apparatus, comprising: an illumination system thatsupplies a projection beam of radiation; a patterning array includingindividually controllable elements that impart the projection beam witha pattern, each of the individually controllable elements include a cellcontaining a polar fluid, a non-polar fluid, an electrode, and a voltagesource; a substrate table that supports a substrate during an exposureoperation; and a projection system that projects the patterned beam ontoa target portion of the substrate.
 2. The lithographic apparatus ofclaim 1, wherein the polar fluid, the non-polar fluid, the electrode,and the voltage source are arranged to selectively apply an electricfield across the cell for voltage-controlled displacement of saidnon-polar fluid within the cell.
 3. The lithographic apparatus of claim2, wherein the polar fluid and the non-polar fluid have substantiallythe same transmissivity, but different refractive indices.
 4. Thelithographic apparatus of claim 1, wherein the polar fluid and thenon-polar fluid have substantially the same transmissivity but differentrefractive indices.
 5. The lithographic apparatus of claim 1, whereinthe relative properties of the polar and non-polar fluids are chosen tochange a phase of part of the projection beam of radiation associatedwith the cell by a predetermined amount relative to other parts of theprojected beam depending on a level of applied voltage by the voltagesource.
 6. The lithographic apparatus of claim 1, wherein the polarfluid is water.
 7. The lithographic apparatus of claim 1, wherein thenon-polar fluid is oil.
 8. The lithographic apparatus of claim 1,wherein the cell comprises a reflective surface on an opposite insidesurface of the cell from a radiation entry surface, such that theprojection beam of radiation travels through the cell twice.
 9. A devicemanufacturing method, comprising: forming each of individuallycontrollable elements in an array of individually controllable elementswith a cell containing a polar fluid, a non-polar fluid, an electrode,and a voltage source; using the array of individually controllableelements to impart a projection beam with a pattern; and projecting thepatterned beam of radiation onto a target portion of a substrate. 10.The method of claim 9, further comprising: controlling a phase of a partof the projection beam associated with each element relative to the restof the projected beam by directing the projection beam through the cellcontaining layers of the polar and the non-polar fluids, which have oneor both of substantially a same transmissivity, but different refractiveindices, or adjustable relative layer thicknesses.